The present invention relates to infrared-transmissive glasses.
In the design of optical systems which are to be operated over broad wavelength bands, it is necessary to have two materials available which possess significantly different optical dispersions. The difference in dispersions can be used as a design parameter to affect the focus position of different wavelengths within the wavelength band, and superior imagery can thus be achieved over systems which are uncorrected for the dispersive properties inherent in all optical materials.
The great majority of infrared optical systems in use today use germanium as a low dispersion infrared optical material. A dispersion difference is achieved by using more highly dispersive materials such as Ge.sub.28 Se.sub.60 Sb.sub.12, Ge.sub.33 Se.sub.55 As.sub.12, zinc sulfide, zinc selenide, or gallium arsenide--to name a few--in conjunction with the germanium. Thus, because of the rapid growth of the infrared industry, and because of the near universal use of germanium in IR optical systems, the utilization of germanium has greatly increased. The resulting demand has caused the price of germanium to escalate rapidly and considerable concern exists regarding its future availability.
Thus it is an object of the present invention to provide a germanium-free low dispersion infrared optical material.
It is a further object of the invention to provide a low-dispersion infrared optical material which does not require materials which are in limited supply.
Chalcogenide glasses, which have been studied extensively for IR transmission, include a wide variety of compounds containing a chalcogenide element (i.e. oxygen, sulfur, selenium, or tellurium). However, oxygen-containing glasses are typically not transmissive at wavelengths of interest. Although some low dispersion chalcogenide glasses are previously known (e.g. arsenic triselenide) the known materials have consistently exhibited undesirable mechanical properties such as low melting point, low modulus of rupture, and low hardness.
Thus it is an object of the present invention to provide a low dispersion chalcogenide glass having good mechanical properties.
It is a further object of the present invention to provide a low dispersion chalcogenide glass having a softening point greater than 250.degree. C.
It is a further object of the present invention to provide a low dispersion chalcogenide glass having good hardness at room temperature.
A further difficulty with prior art compositions is their use of hazardous constituents. In particular, arsenic is violently poisonous and also carcinogenic. The high arsenic vapor pressure over a melt of arsenic--containing mixture poses major safety hazards.
Thus, it is an object of the present invention to provide a low-dispersion chalcogenide glass composition which does not contain carcinogenic or highly poisonous constituents.
It is a further objective of the present invention to provide a low-dispersion chalcogenide glass composition, having good mechanical strength, which does not contain arsenic.
Additional background on wavelength--corrected and infrared optical systems is found e.g., in the OSA Handbook of Optics (1978) and the Infrared Handbook (1978), both of which are hereby incorporated by reference.
It is a further object of the invention to provide an economical wavelength--corrected optical system.